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0-30V/10A Adjustable power supply - Part 2 (Mains section)

When you switch on a big power supply, the initial current drawn from the mains is very high. There are two main reasons for this :
  • Transformers will draw a very heavy current at switch on, until the magnetic flux has stabilised. The effect is worst when power is applied as the AC voltage passes through zero, and is minimised if power is applied at the peak of the AC waveform. This is exactly the opposite to what you might expect.
  • At power on, the filter capacitors are completely discharged, and act as a short circuit for a brief period which may destroy the bridge rectifier.
These phenomena are well known to manufacturers of very high power amps used in PA and industrial applications, but 'soft start' circuits are not commonly used in consumer equipment. Anyone who has a large power amp - especially one that uses a toroidal transformer - will have noticed a momentary dimming of the lights when the amp is powered up. The current drawn is so high that other equipment is affected.



Above image shows the mains section of the power supply. A 16 Ohm/25W resistor has been put in series with the primary winding of the transformer. The resistor is bypassed with a relay which is controlled by the microprocessor. Alternatively, one could use a simple delay circuit. The goal is to bypass the resistor about 100mS after startup. Your rectifier will thank you. Please note, that the resistor will have to be a fairly heavy duty one as the surge current will likely destroy lesser resistors.

Another important factor to keep in mind is that the power supply should never be switched on with a load attached. In this case, that is not a concern, as the output terminals will remain idle until the microprocessor instructs the analog circuitry to start delivering output - obviously not before the startup delay has ended.

Also shown in the schematic is a second transformer. This is a much smaller torroid and will supply all the analog- and digital circuitry in the power supply.

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